The invention relates to a torsional vibration damper operating with a viscous friction medium for limiting the vibration excursions and hence for limiting the stress especially on crankshafts in internal combustion engines. Such vibration dampers are known in a variety of types, and consist basically of a flange body rigidly affixed to the shaft and an inertial body rotatably or resiliently mounted thereon, which forms with corresponding surfaces of the flange body narrow interstices which are filled with a highly viscous liquid. The relative movements which occur between the flange body and the inertial body acting as the inert mass in the case of torsional vibrations result in high shearing forces in the interstices and thus produce the desired damping action.
Depending on the arrangement of the inertial body, a distinction can be made between dampers having an internal torsional mass, in which the flange body is in the form of a casing and completely envelops the inertial ring, and those having an external torsional mass in which seals are necessary between the flange body and the inertial body enveloping the interstices.
Although the last-mentioned embodiment using an external inertial mass would have obvious advantages, consisting in the greatly reduced solid mass of the flange body due to the elimination of the housing and in the accessibility of the inertial body, only the fully enclosed design with its simple construction has hitherto been adopted in practice. Nevertheless, this construction has a number of weaknesses, which are manifesting themselves more and more unpleasantly especially in the case of engines subject to greater stress.
Some of the particular disadvantages are the limitation of the interstitial surface area, which is given by the surface area of the inertial ring, and, as a result thereof, the very narrow interstices which are unavoidable if optimum coupling is to be achieved with the oil viscosities available and, as an additional consequence thereof, the very small volume of the viscous friction medium, in which the entire work of the damper must be transformed into heat.
As it is known, between the above-named three magnitudes, at a given viscosity and a given diameter, the coupling is proportional to the interstitial surface area and inversely proportional to the width of the interstices, while the volume corresponds to the product of the two magnitudes. Thus, for example, a doubling of the interstice surface area over conventional dampers would permit twice the interstice width and result in four times the working volume.
The above-mentioned disadvantages are all thus inter-related and require not only a high surface quality and precision of form, but they also easily result in thermal and mechanical overstressing of the silicone oils used and thus to the ruination of the damper.
Furthermore, the internal pressure occurring during operation as a result of the temperature rise undesirably deforms the housing, resulting in a reduction of the damping action, and also resulting in interference and wearing of the inertial ring against the casing. Another problem is the very precise guidance of the inertial ring by means of the conventional friction bearing which is endangered, especially at high loads and upon the occurrence of axial vibrations, due to the poor lubricating properties of the silicone oils used and the relatively narrow bearing clearance in which the silicone oils might be stressed beyond their shear strength.
The fact that no practical solution has yet been found for a viscosity damper with an external inertial mass is evidently because in this case not are only the above-mentioned disadvantages of the enclosed construction eliminated, but also additional problems have to be solved, which are the result of the additionally required seal between the flange body and the torsional body, and of additional soild sealing surfaces between the individual parts of the two bodies, in view of the considerable creeping ability (low surface tension) of the silicone oils used and the internal pressures that occur. Furthermore, for the practical embodiment, there is the very important problem of finding for this basically more expensive damper construction consisting of more parts, a design which will permit competitively inexpensive manufacture along with no greater external dimensions.
Thus, solutions have been proposed in which, instead of friction seals which are unusable in sustained operation, seals of resilient material are proposed, which are under bias or are vulcanized on, and at the same time are to provide for the precise guidance of the flywheel. Such a construction (described for example in German Auslegeschrift 1295287), however, has the disadvantage that, although the radial and axial rigidity are adequate, the necessary movement in the tangential direction upon the occurrence of torsional vibrations is made possible only by sliding or by severe deformations, both of which would result in difficulties in sustained operation. In another proposal made in German Offenlegungsschrift 2362128, this problem is not solved, either, because in the case of the rubber rings installed by vulcanization, the same comments apply on account of the little amount of space available, while in the case of the likewise proposed friction bearings, a long life of the O-rings cannot be achieved. In short, therefore, the proposed solutions that have become known for the combined mounting in bearings and sealing of the inertial body against the flange body have the disadvantage that they are unable to satisfy the requirements as to greater precision and accordingly rigidity in the radial and axial directions, combined with sufficiently great elasticity in the circumferential direction, and that they furthermore do not have sufficient volume to accommodate the deforming work that occurs in sustained operation.
In addition, no provision is made for the prevention of a rise in the internal pressure when the damper becomes heated, which would be harmful to the seals themselves, or for the protection of the seals against it.
As regards the tight seal between the individual parts of the damper to be mounted, an embodiment in accordance with German Auslegeschrift 1295287 does appear to be feasible, but in this case the interstitial surface area is not greater than it is in the case of conventional enclosed damper constructions. On the other hand, the arrangement shown in German Offenlegungsschrift 2362128 of a plurality of intermeshing disks and intermediate rings permits a certain enlargement of the total interstitial surface area (although it can hardly be utilized at the given external dimensions of the damper due to the division called for here, into an inertial mass part and a damping part), but it achieves this at the cost of the disadvantage of four flat seals per pair of lamins or of correspondingly precisely machined sealing surfaces, which in either case involves higher manufacturing costs.